The present invention relates to a superconductor used in superconducting magnets for nuclear fusion research and the like, and in particular relates to a superconductor of a type forced cooled with a coolant.
Recently, there have been proposed various superconducting coils which use the so-called "hollow" superconductor provided along its center with a coolant channel through which a coolant such as supercritical helium is forced to circulate, so that the superconductor is forcedly cooled from the inside. Typical examples such hollow superconductors are shown in FIGS. 1 to 3. In FIG. 1, superconducting filaments 1 are embedded in the walls of a stabilized member 2 of a rectangular section, made of copper and the like, which has a coolant channel 3 centrally formed through it. FIG. 2 illustrates another type of superconductor in which extremely fine multi-filamentary superconducting wires 4 are wound or twisted around the outer faces of a stabilized member 5 of the same material as in FIG. 1. In another type of superconductor shown in FIG. 3, there are provided four grooves 6 longitudinally formed in the outer faces of the stabilized member 7. Braided and worked superconducting wires 8 are fitted into and soldered to the grooves 6.
Superconducting magnets utilizing superconductors of such forced cooled types are advantageous in that they are uniformly cooled by forcedly circulating a coolant in the superconductor, in that consumption of the coolant is relatively small, and in that the magnetic coil is compact and high in mechanical strength. However, in these superconducting magnets, the superconducting wires are indirectly cooled through the stabilizing member, and hence the efficiency of cooling is relatively low, which causes delay in recovery of superconducting state when it is lost due to a heat spot produced in the superconducting wires.
On the other hand, there has been proposed the so-called "bundle" type superconductor, as shown in FIG. 4, in which a great number of superconducting wires 9 are inserted into a square tube 10 and a coolant such as liquid helium flows the space 11 between the wires. In this superconductor the coolant comes into direct contact with the surfaces of the wires 9 and thereby direct cooling is performed. However, it is rather difficult to make the coolant smoothly flow through the superconductor, and local stay of the coolant is hence produced, resulting in an increase in temperature, which can cause a heat spot to be produced or a delay in recovery of the superconducting state.
In view of the above, it has been proposed in Japanese patent application No. 57-45795 filed Mar. 23, 1982, now Japanese Published Application No. 58-162008 published on Sept. 26, 1983 a superconductor into which advantageous structures of the hollow superconductors and the direct cooling type superconductor shown in FIG. 5 are incorporated. This superconductor has a structure which is excellent in overall cooling efficiency and local stability, and is further capable of withstanding relatively large electromagnetic force.
FIG. 5 shows a typical example of this prior art superconductor, in which a large number of superconducting wires 12 are accommodated in a hollow stabilizing member 13 having a rectangular cross section and made of an electrically highly conductive material at the cryogenic temperature such as copper, copper alloy, high purity aluminum, aluminum alloy, etc. The superconducting wires 12 are made of a superconducting alloy material, such as Nb-Ti alloy and Nb-Ti-Ta alloy, or an intermetallic superconducting material such as Nb.sub.3 Sn, V.sub.3 Ga, Nb.sub.3 Ge, etc. The stabilizing member 13 is surrounded with a casing 14 of a rectangular tube made of copper, stainless steel, titanium alloy, etc. The stabilizing member 13 and the casing 14 are spaced by means of several separators 15 made of stainless steel or the same material as in the stabilizing member 13, and a coolant passage 16 is thereby formed between the stabilizing member 13 and the casing 14. The stabilizing member 13 has a plurality of passages 17 formed through it for flowing the coolant between the inside thereof and the coolant passage 16. The passages 16 may be in the form of a round hole, slot, slit or the like. A coolant, such as supercritical helium, which flows the coolant passages 16 passes through the passages 17 and enters into the space 18 formed between the superconducting wires 12 located inside the stabilizing member 13, where it comes into direct contact with the superconducting wires 12. Thus, a flow of the coolant is generated in the space 18 within the stabilizing member 13.
In the superconductor shown in FIG. 5, superconducting wire assemblies 19A and 19B consisting of a plurality of superconducting wires 12 are superposed with a thin tape 20 of a high resistance conductive material, such as cupronickel, interposed between them, and are accommodated in the stabilizing member 13. The tape 20 keeps the assemblies apart, and thereby prevents coupling current to flow between the assemblies, so that the deterioration in superconductivity characteristics of the superconductor at extremely high energizing speed, as in pulse drive, is prevented. Also, between each superconducting wire assembly and the stabilizing member 13, there is interposed a thin tape 21 similar to the tape 20. The tape 21 serves to prevent coupling current to flow the assemblies 19A and 19B through the stabilizing member 13. The tape 21 covering the superconducting wire assemblies 19A and 19B is provided with openings for allowing the coolant to pass through the passages 17 into the space 18 defined between the superconducting wires 12.
In this superconductor, the total cooling is carried out by a steady state flow of the coolant which flows the coolant passage 16, and thereby uniform cooling is carried out as in the previously-described prior art hollow superconductors. On the other hand, the direct cooling is made by bringing the coolant into direct contact with superconducting wires 12. Thus, this superconductor achieves fairly high cooling efficiency. It is to be noted that the coolant which flows outside and inside the atabilizing member is exchanged through the passages 17, and that there is hence little possibility of any heat spot is produced or the recovery of the superconducting state being delayed due to the local occurrence of a rise in temperature of the coolant as in the prior art direct-cooling-type superconductor in FIG. 4. Thus, the superconductor shown in FIG. 5 is superior also in stability in both the steady state and transient state. Further, this flow of the coolant through the passages 17 achieves sufficient cooling of the superconductor even when the superconducting wires have an assembly structure, such as a braided structure, which allows the coolant to flow less smoothly along the conductor. This fact gives less restriction to the design of the assembly structure of the superconducting wires. When in this superconductor the superconducting state is broken into the electrically normal conducting state due to a certain disturbance, part of the current flows into the stabilizing member, resulting in the production of heat. However, this heat of the stabilizing member is removed by the coolant which flows outside that member, and hence superconductor is capable of recovering the superconducting state soon as compared to the direct cooling system of the bundle type shown in FIG. 4. The superconductor is centrally provided with the superconducting wires, and hence when wound in a magnetic coil, it is less liable to be degraded in characteristics due to the bending stress.
The superconductor according to Japanese patent application No. 57-45795 is suitable for use in nuclear fusion reactors, electric machines, energy storage apparatus, magnetic resonance device, magnetic separation devices, etc., and particularly in large scale high magnetic field magnets. Further, it is particularly suitable for the superconductor carrying large pulse current.
However, in developing this superconductor for practical use, the inventors have found that it has disadvantages described below. Ordinarily, this kind of superconductor is wound in the shape of a coil into a superconducting magnet so that the edgewise sides C and D thereof are placed perpendicularly to the central axis O. The stabilizing member 13 is rather thick, and hence this superconductor is as a whole high in rigidity. Thus, it is rather difficult to wind it in a coil. In practice, the stabilizing member 13, as shown in FIGS. 7 and 8, consists of a channel member 22 and a planar member 23 fixedly fitted to the channel member 22, the channel member 22 forming the three sides 13A, 13B and 13C, and the planar member 23 the other flatwise side 13D. The planar member 23 is fixed to the channel member 22 by spot welding its contact edges in order to prevent the planar member 23 from falling within the channel member in the winding of a coil, resulting in providing a damage to the superconducting wires. However, this spot welding is laborious and time-consuming, and further the spot welded portions can be separated when the stabilizing member is bent by any unduely large force in the coil winding. FIGS. 9 and 10 show a similar stabilizing member 24 which is different from the stabilizing member 13 in FIGS. 7 and 8 in fitting flanges 26 and 27 extending along the opposite edges of the planar member 25. The planar member 25 is fixed to the channel member 28 with the free ends of the channel member being fitted to the shoulders of the fitting flanges 26 and 27. Also in this case, the channel member 28 and the planar member 25 are put together by spot welding.